The human lymphocyte response to mitogens and HLA antigens will be defined using direct cell counting in combination with 3HTdR incorporation. The interaction of lymphocyte subpopulations with themselves and with mononuclear phagocytes will be measured. Culture conditions will be used in which lymphocytes have been shown, by direct cell counting, to proliferate exponentially with a doubling time of 20 hrs for several days when stimulated with PHA and HLA antigens. Using improved culture techniques designed in our laboratory, we will measure lymphocyte cell cycle parameters, recruitment into cycle, their proliferative capacity and their differentiation kinetics after stimulation by mitogens and HLA antigens. The differentiated effector cells will also be studied to determine if they are cytotoxic lymphocytes and if they can act as suppressor cells. These investigations will provide important information about the proliferative capabilities of lymphocytes as well as to identify artifacts introduced by in vitro methodology. The combined results will provide important insights into the regulation of these responses at the cellular level.